The present invention relates to transmission from a base station to a receiver.
It can apply to a base station of a radiocommunication network, such as a GSM (“Global System for Mobile communications”) or a UMTS (“Universal Mobile Telecommunication System”) network for instance. Of course, it can also apply to other types of base stations.
The receiver can be a mobile terminal, but also any other type of receiver, such as another base station, a base station controller, a radio network controller, etc.
It is known that communication by radio between a base station and a mobile terminal for instance, is subject to phenomena that disturb the radio transmission between the antenna of the base station and the antenna of the mobile terminal, in particular to channel fadings due to destructive interference between signals which follow different propagation paths between the base station and the terminal.
The diversity of one of the characteristics related to this transmission is one of the methods developed for alleviating fading. Thus, use is made of transmission diversity, consisting in equipping the base station with several antennas transmitting the same signals, polarization diversity, frequency diversity (see for example the work “Réseaux GSM” [GSM networks] by X. Lagrange et al, published by Hermes Science Publications, 2000, page 161), etc.
It is known practice to use antennas comprising devices for altering the radiation pattern. Such adjustments pertain for example to the direction of transmission of the antenna or the width of the main transmission lobe.
These alterations of the radiation pattern may be mechanical, such as the orienting of an antenna arranged on an articulated support, mixed electrical/mechanical (cf. patent U.S. Pat. No. 6,198,458), or else purely electronic, as in FR-A-2 792 116 or its U.S. equivalent U.S. Pat. No. 6,480,154.
Most antennas with electronic steering of the beam are composed of several antenna elements individually fed with signals obtained by phase shifting an initial signal. The value of the phase shift is determined as a function of the antenna element to which the phase-shifted signal is addressed, and the direction of transmission by the antenna results from the combining of the mutually phase-shifted signals transmitted by all the antenna elements.
Such antennas, also known as “smart antennas”, are sometimes used to focus a radio beam intended for a particular terminal. In a particular embodiment, the components of an uplink radio signal transmitted by the terminal and which are picked up by the various antenna elements, are analyzed in terms of phase shifts so as to estimate a direction in space from which this uplink signal originates. Corresponding phase shifts are then applied to the downlink signal intended for this terminal so that its transmission is oriented in this direction. Such electronic steering of the beam allows considerable reductions in interference level.
A base station with such antenna is described below with reference to FIG. 1 and FIG. 2.
In FIG. 1, a base station 100 transmits by means of the antenna 1, a radio signal intended for a terminal 200 situated within range of this antenna. In this example, the antenna 1 consists of juxtaposed radiating elements 2. All these radiating elements 2 are fixed with respect to the support of the antenna 101, and oriented facing the geographical sector intended to be served by the antenna.
The transmission pattern of the antenna generally consists of a main lobe, corresponding to an angular sector inside which the radiation power is greater than a fixed value, and limited according to the separation with respect to the antenna by the reduction in power related to the propagation of the radiation. The axis of this main lobe corresponds to the direction D of transmission of the antenna 1.
The direction D of transmission can be charted by a system of spherical coordinates having as pole the centre O of the antenna 1. These coordinates comprise for example the angle of elevation of the direction D of transmission with respect to a horizontal plane containing the point O, and the angle of azimuth between the projection of the direction D onto the horizontal plane and a reference axis R contained in this plane, for example oriented perpendicularly to the grouping of radiating elements and passing through the point O.
Fluctuations in the direction D of transmission of the antenna 1, e.g. due to movement of the mobile terminal 200, are then charted through the evolution of the angles of elevation and of azimuth. Thus, a fluctuation in the direction D lying in a vertical plane corresponds to a variation in the angle of elevation. A fluctuation lying in a horizontal plane corresponds to a variation in the angle of azimuth.
In most digital radiocommunication systems, the signals are transmitted after application of a channel coding and of an interleaving. The channel coding adds redundancy to the symbols of the digital signal, with a structure allowing the receiver to detect and correct the transmission errors. The codes customarily employed have optimal performance when the errors arising in the course of transmission are uncorrelated. The interleaving consists of a permutation of the symbols that is intended to tend towards this condition of non correlation while the transmission errors on a radio interface have a tendency rather more to arise through packets on account of the fading phenomenon. The permutation of the interleaving pertains to a certain duration (of a few tens of milliseconds) chosen to achieve a compromise between the performance of the decoder and the processing delay which the interleaver entails. This interleaving duration may vary from one channel to another, such as for example in the case of a UMTS (“Universal Mobile Telecommunication System”) system where it is from 10 to 80 ms.
FIG. 2 diagrammatically shows an example of the means employed by a base station to adapt the antenna pattern, in order to focus it in a particular direction of transmission. Each signal component S1, S2, . . . SM, intended for a particular terminal 200 or one belonging to a common channel, is produced by a processing pathway comprising a channel coder 3, an interleaver 4, a modulator 5, then a power adjustment module 6. The signal components S1, S2, . . . , SM, delivered by the various processing pathways are subsequently combined by a multiplexing unit 7 into a baseband signal S delivered to the radio transmission stage.
The makeup of the modulators 5 and of the multiplexing unit 7 depends on the multiple access mode employed in the radiocommunication system to which the invention is applied. In a system where the multiple access is by time division (TDMA), as for example GSM, the modulators 5 carry out the modulation in baseband or on an intermediate frequency, whereas the multiplexer 7 distributes the signal components S1, S2, . . . , SM, into respective time slots of the signal frames, corresponding to the various channels. In a system where the multiple access is by code division (CDMA), such as for example UMTS, the modulators 5 can carry out the spectrum spreading by applying the spreading codes assigned to the various channels, whereas the multiplexer 7 simply performs a summation of the signal components S1, S2, . . . , SM.
In the radio stage, a separator 8 reproduces the signal S on each transmission pathway corresponding to a radiating element 2 of the antenna 1. The phase-shifting unit 9 then applies a respective phase shift D1, D2, . . . , DN to the signal of each transmission pathway. Each phase shift is determined by the position in the antenna 1 of the radiating element 2, and depends on the direction of transmission of the antenna 1 controlled by the transmission pattern controller 10. FR-A-2 792 116 describes an exemplary phase adaptation device usable as a phase-shifting unit 9.
The radio stage subsequently undertakes the conventional operations of filtering, of conversion to analogue 11, of transposition to the carrier frequency 12 and of power amplification 13 on the basis of the signals delivered by the phase-shifting unit 9. Each radiating element 2 then receives from the amplifier 13 associated with it, by way of a duplexer 14, the phase-shifted radio signal E1, E2, . . . , EN corresponding to its transmission pathway.
Alternatively, the phase shifts could be applied to the signal of each transmission pathway in an analogue way, i.e. after conversion to analogue 11.
The phase-shifting unit 9 can also perform a weighting of the amplitude of the signal corresponding to each transmission pathway. In a manner known to the specialist in radio transmissions, such a weighting, jointly with phase shift law applied, makes it possible to modify a width of the transmission pattern by altering the amplitudes of the signals transmitted by each radiating element 2. Thus, during the transmission of the signal by the antenna 1, the angular aperture of the transmission pattern can be modified simultaneously with the fluctuation of the direction of transmission D.
An important advantage of such smart antennas is to make it possible to maximize, for a given mobile terminal, the signal-to-interference ratio, by creating an antenna pattern whose “zeros”, that is to say locations with very weak transmission or reception power, are in the direction of the interferers of the mobile terminal in question.
This principle is illustrated in FIG. 3 where two terminals 21 and 22 are in communication with the base station 20. The lobes 23 and 24 of the antenna of the base station 20 are oriented mainly towards the two terminals 21 and 22 respectively. The figure clearly shows that the signal-to-noise ratio is maximized for each of the two terminals since the overlap of the lobes 23 and 24 is limited.
Theoretically and ideally, i.e. when the mobile terminals are uniformly distributed around the base station and the angular spreading of the multiple paths is negligible, the use of a smart antenna as described above can reduce the interference level by the number of main lobes (e.g. by 2 in the example of FIG. 3).
Despite the interference reduction they imply, there is a problem with smart antennas in that they also reduce the number of multiple paths each mobile terminal can receive. Indeed, since they transmit mainly in one lobe with respect to a particular mobile terminal, they do not generate significant multiple paths in directions outside of the angular width of said lobe. The angular diversity is thus reduced.
An object of the present invention is to overcome this angular diversity reduction.
Another object of the invention is to alleviate fading, possibly while ensuring interference reduction simultaneously.